Expansion tank

Water expands as it heats, and in a closed system that pressure has to go somewhere. An expansion tank gives it a cushion, protecting the water heater, boiler, and piping from pressure spikes. Code in most Eastern Iowa jurisdictions requires one on new water heater installs, and a waterlogged expansion tank is a common cause of a dripping relief valve.

Why a closed plumbing system needs somewhere for pressure to go

Modern homes have check valves or pressure-reducing valves at the meter, which makes the plumbing a closed loop: when the water heater fires, the expanding water cannot push back into the city main. Without a cushion, every heating cycle spikes the pressure against your pipes, fixtures, and the tank itself. The expansion tank's air bladder absorbs those spikes hundreds of times a day. That is why inspectors look for one on new water heater installs, and why a missing or dead tank quietly shortens the life of everything downstream.

How to tell an expansion tank has failed

The classic symptom is the water heater's temperature-and-pressure relief valve dripping or spitting after hot water use — pressure spikes are lifting it, which means the cushion is gone. You can check the tank yourself: knock on it top and bottom. A healthy tank sounds hollow on top where the air charge lives; one that sounds dull and full throughout is waterlogged. A failed bladder or lost air charge is a straightforward, inexpensive fix — far cheaper than the relief valve, water heater, and fittings that chronic pressure spikes will take out over time. Expansion tanks on boilers fail the same way and announce it as pressure creeping up on the gauge each time the system heats — worth a glance at every fall tune-up.